A group of pro-Russia activists has stormed the police headquarters in the southern Ukrainian city of Odessa and released scores of prisoners detained at the scene of deadly violence on Friday that culminated in people being burned alive in a trade union building.

In the pouring rain, men armed with clubs battered their way into the building through a vehicle entrance.

Ranks of riot police offered no resistance. When crowds burst into the compound and began smashing windows and wrecking police vans, officers agreed to release the activists.

Men and women, many in tears, emerged from the door of their cell block and left through a tunnel of cheering supporters. Local police said later that 67 people were set free.

"The police did not interfere," said Maksim, 26, an activist wearing a balaclava and a helmet who was one of the first to get inside the compound. "They are only defending their weapons."

Residents and government officials in Kiev have criticised the police, accusing them of fatal passivity during the violence in the Black Sea city.

More than 40 people were killed on Friday during street battles that escalated until pro-Ukraine activists launched a full-scale assault on the trade union building. It was defended by people opposed to the current regime and in favour of closer ties with Moscow.

The Ukrainian prime minister, Arsniy Yatsenyuk, told the BBC that Odessa's security forces were responsible for the deaths.

"I personally blame the security services and law enforcement office for doing nothing," he said. "[They] are inefficient and they violated the law."

Yatsenyuk, who was reportedly due to visit Odessa yesterday, also said that the region's police chief had been dismissed.

Those inside the compound said that there was a lot of sympathy for the pro-Russia cause among lower-ranking police offers. "The police are with us," said Mikhail, 21, who had just been released.

One activist said that he had personally witnessed how police had covered pro-Russia fighters during street battles on Friday.

As prisoners were set free, the crowd chanted: "Odessa is a Russian city" and "Heroes, heroes, heroes!". Many expressed outrage that those who had escaped being burned alive in the trade union building had ended up in jail, while their attackers had not been arrested.

"The [event on] 2 May was a genocide of Russian people," said Timur, 28, who joined in the attack on the police station.

The trade union building was opened to the public on Sunday. Detritus from Friday's battle, including baseball bats, helmets, shields, petrol canisters and children's nappies apparently used to bandage wounds, were scattered around it. Broken glass covered the floor and pools of dried blood were still visible in several rooms.

Kristina, an employee who had been allowed back into the building to clear up her office on the second floor, said that desperate activists had smashed double-glazed windows and made a rope from curtain blinds to try to escape. "There was no fire here," she said. "They were scared of those shooting."

The main stairwell of the five-storey Soviet building was badly burned, as were dozens of offices, but large parts of the building remained intact.

The picture that emerged was one of complete chaos. Pro-Russia defenders, terrified of their attackers, were burned to death or died of asphyxiation just metres from rooms never touched by fire or smoke.

Others said that the construction of the building could have aided the spread of poisonous fumes produced in the conflagration.

Aleksei, a computer programmer who worked in one of the corner offices, said that burning plastic doors on the ground floor, where the fires were concentrated, could have released toxic gases.

"The main stairwell worked as a chimney. Somebody smashed windows on the fifth floor and the air was drawn upwards," he said. Police had allowed him in to check his computer equipment on Saturday, and he said he had seen charred corpses clustered around the main stairwell.

Back at the police compound, Vladimir, who was in tears after his release, said that he had been inside the trade union building during the fighting and as the fires took hold.

He described the utter confusion as pro-Russia activists erected makeshift barricades in corridors in an attempt to hold back their attackers. Along with a group of about 30 other people, Vladimir said he had barricaded himself on the building's roof despite the fear of snipers. They were later rescued by police.